Matrix-stages solid state ultrafast switch

ABSTRACT

A semiconductor switching device for switching high voltage and high current. The semiconductor switching device includes a control-triggered stage and one or more auto-triggered stages. The control-triggered stage includes a plurality of semiconductor switches, a breakover switch, a control switch, a turn-off circuit, and a capacitor. The control-triggered stage is connected in series to the one or more auto-triggered stages. Each auto-triggered stage includes a plurality of semiconductor switches connected in parallel, a breakover switch, and a capacitor. The control switch provides for selective turn-on of the control-triggered stage. When the control-triggered stage turns on, the capacitor of the control-triggered stage discharges into the gates of the plurality of semiconductor switches of the next highest stage to turn it on. Each auto-triggered stage turns on in a cascade fashion as the capacitor of the adjacent lower stage discharges or as the breakover switches of the auto-triggered stages turn on.

FIELD OF THE INVENTION

The present invention relates to a solid state switch and, more specifically, to a solid state switch having a plurality of switch levels connected in series, each level having a plurality of semiconductors connected in parallel and to a method of triggering the solid state switch.

BACKGROUND OF THE INVENTION

Switching high current and high voltage is required in some applications in military, medical, and commercial devices and systems. Depending upon on the application, a switching device may be required to switch tens of kilovolts and tens of kiloamperes. Devices for switching such high current and high voltage have been proposed to include a plurality of levels connected in series, each level having a plurality of high-power semiconductor switching devices connected in parallel. Each semiconductor includes a gate and a driver of the gate. To control these devices, the gate driver has special requirements such as high voltage isolation between levels, minimum delay time in gate pulses from one level to another, overvoltage protection, and sharing voltage protection.

Many drive circuits use pulse transformers in the gate circuitry to isolate one level of the switch from another. Because the switch comprises many semiconductors in series and in parallel, these pulse transformers become very large, costly, and impractical. In addition pulse transformers must be shielded to avoid external magnetic field pick up which could create unwanted low level gate pulses and, as a result, cause misfiring, which may destroy devices connected to the switch.

In response, methods have been developed using power stored in a capacitor floating with the device for the trigger energy. These methods use low-power triggers for a low-power solid state device that discharges the capacitor into the gate of the high-power semiconductor switching device. While still requiring a pulse transformer, because of the lower energy requirements in the gate circuitry, the switching device can be smaller. General examples of these switching devices are described in U.S. Pat. Nos. 5,444,610 and 5,646,833.

Other methods for switching the high-power semiconductor switching devices have been proposed. U.S. Pat. Nos. 6,396,672 and 6,710,994 describe a power electronic switch circuit that includes a silicon-controlled rectifier and a gate trigger circuit coupled to the gate of the silicon-controlled rectifier (SCR). A snubber capacitor is coupled to the anode and cathode of the SCR. Energy stored in the snubber capacitor provides the necessary energy to power the gate trigger circuit to trigger the SCR.

U.S. Pat. No. 6,624,684 describes a compact method for triggering thyristors connected in series using energy stored in a pulse forming network coupled to the gate of each thyristor. Each pulse forming network is coupled to a snubber circuit that, together with the pulse forming network, acts as a snubber capacitor to limit the dv/dt imposed on the thyristor, thereby preventing spurious turn-on of the thyristor. The pulse forming network provides current to the gate of the thyristor through a gate switch to turn on the thyristor while the snubber circuit provides a source of fast rising current to the anode of the thyristor to speed up turn-on as it discharges through the anode of the thyristor. Either a low-power electrical signal through a pulse transformer or an optical signal can be used to trigger the gate switch.

U.S. Pat. Nos. 5,933,335 and 5,180,963 provide examples of an optically triggered switch. In U.S. Pat. No. 5,180,963, there is described an optically triggered solid state switch. The switch uses an optical signal for each set of two high power solid state devices. The optical signal triggers a phototransistor, which in turn triggers a low power solid state device. The low power solid state device then discharge a capacitor through a pulse transformer, producing signals in the gates of two high power solid state devices to turn on the devices.

U.S. Pat. No. 7,072,196 describes a method of turning on a high voltage solid state switch that comprises a set of solid state devices, such as thyristors, connected in series. The switch comprises a snubber circuit coupled to the anode and cathode of each solid state device to speed up turn-on.

SUMMARY OF THE INVENTION

According to an exemplary aspect of the present invention there is provided a semiconductor switching device having a control-triggered stage and one or more auto-triggered stages connected in series. The control-triggered stage includes one or more semiconductor switches connected in parallel, a breakover switch comprising a first end and a second end, and a control switch connected across the breakover switch. Each semiconductor switch includes a power input, a power output, and a control input. The first end of the breakover switch is coupled to the power input of each semiconductor switch, and the second end of the breakover switch is coupled to the control input of each semiconductor switch. Each auto-triggered stage includes one or more semiconductor switches connected in parallel and a breakover switch comprising a first end and a second end. Each semiconductor switch of each auto-triggered stage includes a power input, a power output, and a control input. The first end of the breakover switch of the each auto-triggered stage is coupled to the power input of each semiconductor switch of the each auto-triggered stage, and the second end of the breakover switch of the each auto-triggered stage is coupled to the control input of each semiconductor switch of the each auto-triggered stage. The control-triggered stage is connected in series with the one or more auto-triggered stages.

According to another exemplary aspect of the present invention there is provided a high voltage circuit having an energy storage device, a load, a power supply coupled across the energy storage device, and a semiconductor switching device for coupling the energy storage device across the load. The semiconductor switching device includes a control-triggered stage and one or more auto-triggered stages connected in series. The control-triggered stage includes one or more semiconductor switches connected in parallel, a breakover switch comprising a first end and a second end, and a control switch connected across the breakover switch. Each of the semiconductor switches includes a power input, a power output, and a control input. The first end of the breakover switch is coupled to the power input of each semiconductor switch, and the second end of the breakover switch is coupled to the control input of each semiconductor switch. Each auto-triggered stage includes one or more semiconductor switches connected in parallel and a breakover switch comprising a first end and a second end. Each semiconductor switch of each auto-triggered stage includes a power input, a power output, and a control input. The first end of the breakover switch of the each auto-triggered stage is coupled to the power input of each semiconductor switch of the each auto-triggered stage, and the second end of the breakover switch of the each auto-triggered stage is coupled to the control input of each semiconductor switch of the each auto-triggered stage. The control-triggered stage is connected in series with the one or more auto-triggered stages.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustration, there are shown in the drawings certain embodiments of the present invention. In the drawings, like numerals indicate like elements throughout. It should be understood, however, that the invention is not limited to the precise arrangements, dimensions, and instruments shown. In the drawings:

FIG. 1 illustrates an exemplary embodiment of a system for providing pulsed power to a load, the system comprising a power source, an energy storage device, and a semiconductor switching device, in accordance with an exemplary embodiment of the present invention;

FIG. 2 illustrates an exemplary embodiment of the semiconductor switching device of FIG. 1, in accordance with an exemplary embodiment of the present invention;

FIG. 3 illustrates exemplary waveforms for voltage and current provided by the energy storage device of FIG. 1 during turn-on of the semiconductor switching device, in accordance with an exemplary embodiment of the present invention; and

FIG. 4 illustrates exemplary waveforms for voltage and current provided by the energy storage device of FIG. 1 during turn-off of the semiconductor switching device, in accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As described above, various conventional high voltage, high current solid state switches have been proposed. Some of the conventional high voltage, high current solid state switches comprise one or more solid state devices, such as thyristors, across which a snubber circuit is coupled. The snubber circuit facilitates turn-on of the solid state device but limits using an insulated-gate bipolar transistor (IGBT) or a metal oxide field effects transistor (MOSFET) as the solid state devices because IGBTs and MOSFETs are normally used in snubberless circuits.

Other conventional high voltage, high current solid state switches use pulse transformers to drive the one or more solid state devices. Pulse transformers, however, are not desirable as they increase the inductance of the solid state switches. Further, the primary winding wire of the trigger transformer conducts the same current that the one or more solid state devices conduct. With wide pulse duration and high load current, the primary winding wire becomes very thick, and the trigger transformer becomes very big and insufficient.

The conventional solid state switches which use snubber circuits or pulse transformers are disadvantageous for additional reasons. Typically, a solid state switch operates in a very narrow range of voltage, usually between V_(switch)-V_(switch)/N_(levels) and V_(switch). The value of the snubber parameters and the design of the trigger transformer must be selected to avoid auto triggering of the switch during the charging phase. Unwanted turn-on may happen if any unexpected voltage spikes in initial dv/dt occur.

In all of the above-mentioned switches, pulse transformers, multiple phototransistors, and other components are required for each high power solid state device being triggered. With many semiconductors in series and in parallel, the multitude of triggering devices required become very large, thereby increasing the cost, size, and expense of the solid state switch. The size and expense may become impractical if the number of solid state devices is large. Furthermore, if pulse transformers are use, the pulse transformers must be shielded to avoid external magnetic field pick-up, which could create unwanted low-level gate pulses and, as a result, cause misfiring and destroy the solid state devices. Shielding further increases the size and expense of the solid state switch.

Referring now to FIG. 1, there is illustrated a system 100 for use in a pulsed power application, in accordance with an exemplary embodiment of the present invention. The system 100 comprises a power source 110, an energy storage device 120, a load 130, and a switch 200. The power source 110 is coupled to the energy storage device 120 and to ground 140. The switch 200 is coupled to the energy storage device 120 and to ground 140. The load is coupled to the energy storage device 120 and to ground 140.

During use of the system 100, the switch 200 is initially open, and the power source 110 charges the energy storage device 120. When the energy storage device 120 has been charged to a desired level, the power source 110 may be disabled or disconnected from the device 120.

When power is desired to be delivered to the load 130, the switch 200 is closed, and the charge stored in the energy storage device 120 discharges into the load 130. The charging time of the energy storage device 120 may be two or more orders of magnitude greater than the discharge time thereof. Thus, the current supplied to the load via the switch 200 may be many times that of the current supplied to the energy storage device 120 during charge up by the power source 110. Exemplary embodiments of the energy storage device 120 include one or more capacitors, one or more transmission lines, or a pulse forming network. Exemplary values of the voltage and current provided by the energy storage device 120 during discharge are, respectively, tens of kilovolts and ten of kiloamperes, making the system 100 a high voltage, high current circuit.

In the system 100, each of the power source 110, the switch 200, and the load 130 is connected to ground 140. It is to be understood from FIG. 1 that alternative embodiments of the system 100 are contemplated. For example, in an exemplary alternative embodiment, the positions of the switch 200 and the energy storage device 120 are switched.

Illustrated in FIG. 2 is a circuit diagram for the switch 200 of FIG. 1, in accordance with an exemplary embodiment of the present invention. The switch 200 is used for switching a current I_(S) provided by the energy storage device 120 at a voltage V_(S). As noted above, the current I_(S) may be tens of kiloamperes, and the voltage V_(S) may be tens of kilovolts.

The switch 200 comprises a plurality of levels or stages, respectively designated as 210 ₁, 210 ₂, . . . , 210 _(n), connected in series. In the exemplary embodiment of the switch 200 illustrated in FIG. 2, there are n stages 210 ₁ through 210 _(n). By using a plurality of stages 210 ₁ through 210 _(n), the switch 200 divides the source voltage V_(S) over n stages so that V_(S)/n is less than the maximum permissible voltage across any of the stages 210 ₁ through 210 _(n). Thus, the switch 200 has a higher voltage hold-off capability than it would have had it had only one stage, and each stage is protected from overvoltage.

Each stage comprises a plurality of solid state semiconductor switches or devices, respectively designated as S_(x,y), connected in parallel, where x refers to any of stages 210 ₁ through 210 _(n) and y refers to the semiconductor devices in each stage. In the exemplary embodiment of the switch 200 illustrated in FIG. 2, the first stage 210 ₁ comprises m semiconductor devices S_(1,1) through S_(1,m) connected in parallel (herein, “S₁” is used as shorthand to refer to the plurality of semiconductor devices S_(1,1) through S_(1,m) in the stage 210 ₁); the second stage 210 ₂ comprises m semiconductor devices S_(2,1) through S_(2,m) connected in parallel (herein, “S₂” is used as shorthand to refer to the plurality of semiconductor devices S_(2,1) through S_(2,m) in the stage 210 ₁); and the nth stage 210 _(n) comprises m semiconductor devices S_(n,1) through S_(n,m) connected in parallel (herein, “S_(n)” is used as shorthand to refer to the plurality of semiconductor devices S_(n,1) through S_(n,m) in the stage 210 _(n)). By using a plurality of m semiconductor devices in each stage, the switch 200 divides the input current I_(S) by m. Thus, each stage 210 ₁ through 210 _(n) and, specifically, each semiconductor device S_(x,y) is protected from overcurrent.

In the exemplary embodiment illustrated in FIG. 2, the first stage 210 ₁ is a control-triggered stage used to control the turn-on and turn-off of the stages 210 ₂ through 210 _(n) and thereby of the switch 200. FIG. 2 illustrates that the control-triggered stage 210 ₁ is the first or bottommost stage and connected to ground 140. It is to be understood that the control-triggered stage 210 ₁ need not be so located and could be positioned in the place of any of the other stages 210 ₂ through 210 _(n) in the switch 200.

The control-triggered stage 210 ₁ comprises the semiconductor devices S_(1,1) through S_(1,m) having respective control inputs CI_(1,1) through CI_(1,m), power inputs PI_(1,1) through PI_(1,m), and power outputs PO_(1,1) through PO_(1,m). The power outputs PO_(1,1) through PO_(1,m) are connected to an output 211 ₁ of the control-triggered stage 210 ₁ which is at ground 140. The control-triggered stage 210 ₁ further comprises a capacitor C₁, a turn-off circuit 215, and a suppressor Z₁. A first side or end of the suppressor Z₁ is connected to the output 211 ₁, as is a first side or end of the capacitor C₁ and a first end 215A of the turn-off circuit 215. A second side or end of the capacitor C₁ is connected to the control inputs CI_(2,1) through CI_(2,m) of the semiconductor devices S_(2,1) through S_(2,m) of the second stage. A second side or end of the suppressor Z₁ is connected to the control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m), as is a second end 215B of the turn-off circuit 215.

The control-triggered stage 210 ₁ further comprises a breakover switch BOS₁, a second side or end of which is coupled to the second side of the suppressor Z₁. A first side or end of the breakover switch BOS₁ is coupled to the power inputs PI_(1,1) through PI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m) via a resistor R₁. The power inputs PI_(1,1) through PI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m) are connected to an input 212 ₁ of the control-triggered stage 210 ₁. A switch SW₁ is disposed in the control-triggered stage 210 ₁ across the breakover switch BOS₁.

In an exemplary embodiment, the suppressor Z₁ is a Zener diode, the breakover switch BOS₁ is a breakover diode, the switch SW₁ is a MOSFET, and the turn-off circuit 215 is a MOSFET. The first side of the Zener diode Z₁, its anode, is connected to the output 211 ₁ of the stage 210 ₁, and the second side of the Zener diode Z₁, its cathode, is connected to the control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m). The first side of the breakover switch BOS₁, its anode, is connected to the resistor R₁, and the second side of the breakover switch BOS₁, its cathode, is connected to the control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m).

In this exemplary embodiment, the collector of the MOSFET SW₁ is connected to the anode of the breakover diode BOS₁, and the emitter is connected to the cathode of the breakover diode BOS₁. The gate of the MOSFET SW₁ serves as a control input to selectively turn on the switch 200. The collector of the MOSFET 215 is connected to the control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m), and the emitter is connected to the output 211 ₁ of the control-triggered stage 210 ₁. The gate of the MOSFET 215 serves as a control input to selectively turn off the switch 200. The control inputs into the MOSFETS SW₁ and 215 are low-power inputs, which allows for low-power control of the switch 200.

Still referring to FIG. 2, the second through nth stages, 210 ₂ through 210 _(n), are auto-triggered stages triggered by the control-triggered stage 210 ₁. The stages 210 ₂ through 210 _(n) are constructed similarly to the stage 210 ₁ but differ in that they do not include the switch SW₁ or the turn-off circuit 215.

The second stage 210 ₂ comprises the semiconductor devices S_(2,1) through S_(2,m) having respective control inputs CI_(2,1) through CI_(2,m), power inputs PI_(2,1) through PI_(2,m), and power outputs PO_(2,1) through PO_(2,m). The second stage 210 ₂ further comprises a capacitor C₂, a suppressor Z₂, a breakover switch BOS₂, and a resistor R₂. The power outputs PO_(2,1) through PO_(2,m) of the semiconductor devices S_(2,1) through S_(2,m) are connected to an output 212 ₁ of the second stage 210 ₂. The output 211 ₂ of the second stage 210 ₂ is the input 212 ₁ of the first stage 210 ₁. Thus, the second stage 210 ₂ is connected in series to the first stage 210 ₁.

A first side or end of the suppressor Z₂ is connected to the output 211 ₂ of the second stage 210 ₂, as is a first side or end of the capacitor C₂. A second side or end of the suppressor Z₂ is connected to the control inputs CI_(2,1) through CI_(2,m) of the semiconductor devices S_(2,1) through S_(2,m) and to a second side or end of the breakover switch BOS₂. A first side or end of the breakover switch BOS₂ is coupled to the power inputs PI_(2,1) through PI_(2,m) of the semiconductor devices S_(2,1) through S_(2,m) via the resistor R₂. A second side or end of the capacitor C₂ is connected to the control input of the semiconductor devices of the next higher stage. The power inputs PI_(2,1) through PI_(2,m) of the semiconductor devices S_(2,1) through S_(2,m) are connected to the input 212 ₂ of the auto-triggered stage 210 ₂, which input 212 ₂ is an output 211 ₃ of the next higher stage.

Each next higher stage through to stage n-1 is configured the same as the stage 210 ₂ (stage 2). The last stage, stage 201 _(n) comprises the semiconductor devices S_(n,1) through S_(n,m) having respective control inputs CI_(n,1) through CI_(n,m), power inputs PI_(n,1) through PI_(n,m), and power outputs PO_(n,1) through PO_(n,m). The stage 201 _(n) further comprises a capacitor C_(n), a suppressor Z_(n), a breakover switch BOS_(n), and a resistor R_(n). These components are connected in the same manner as those in the auto-triggered stage 210 ₂, except that the second side or end of the capacitor C_(n) is not connected to the control input of the semiconductor devices of the next higher stage as there is no next higher stage. Rather, the second side or end of the capacitor C_(n) is connected to the input 212 _(n) of the auto-triggered stage 210 _(n). This input 212 _(n) is connected to an input 220 of the switch 200. As with the other auto-triggered stages, the output 211 _(n) of the auto-triggered stage 201 _(n) is connected to the input of the next lowest stage, in this case an input 212 _(n-1) of the auto-triggered stage 210 _(n-1) (not illustrated).

In an exemplary embodiment, for each of the stages 210 ₂ through 210 _(n), the suppressors Z₂ through Z_(n) are Zener diodes, and the breakover switches BOS₂ through BOS_(n) are breakover diodes. The first side of each of the Zener diodes Z₂ through Z_(n), its anode, is connected to the respective output 211 ₂ through 211 _(n) of the respective stage 210 ₂ through 210 _(n), and the second side of each of the Zener diodes Z₂ through Z_(n), its cathode, is connected to the respective control inputs of the respective semiconductor devices S₂ through S_(n). The first side of each of the breakover diodes BOS₂ through BOS_(n), its anode, is connected to the respective resistor R₂ through R_(n), and the second side of each of the breakover diodes BOS₂ through BOS_(n), its cathode, is connected to the respective control inputs of the respective semiconductor devices S₂ through S_(n).

As further shown in FIG. 2, the breakover switches BOS₁ through BOS_(n) are identical, as are the respective components among the stages 210 ₁ through 210 _(n). The breakover switches BOS₁ through BOS_(n) have a breakover voltage equal to V_(S)/n+ΔV, where V_(S) is the maximum voltage seen across the switch 200, n is the number of stages, and ΔV is a minimum acceptable difference in blocking voltage between a stage and its breakover switches. The breakover voltage of each of the breakover switches BOS₁ through BOS_(n) is desirably ΔV greater than V_(S)/n, so that the switch 200 does not automatically turn on when V_(S) is applied at its input 220. For example, if the switch 200 comprises 5 levels, and V_(S) is 5,000V, the maximum voltage across each stage is 1,000V, and each breakover switches BOS₁ through BOS_(n) desirably has a breakover voltage greater than 1,000V. For example, the breakover voltage may be 1,007V so that ΔV=7V.

Turn-on and turn-off of the control-triggered stage 210 ₁ is now described. In its initial state, the switch 200 is open and a voltage V_(S) is present at its input 220 by the energy storage device 120. The voltage drop across each stage is equal to V_(S)/n, and each capacitor C₁ through C_(n) is fully charged. Each breakover diode BOS₁ through BOS_(n) desirably has a breakover voltage greater than V_(S)/n but less than V_(S)/(n-1), so that once one stage turns on, all stages turn on. The voltage drop V_(S)/n across each stage is distributed over the suppressor, the breakover switch, and the resistor of each respective stage.

Turn-on of the control-triggered stage 210 ₁ and the auto-triggered stages 210 ₂ through 210 _(n) is now described. In the description below, the voltage across each stage 210 ₁ through 210 _(n) is, respectively, ΔV_(S1) through ΔV_(Sn), each of which is equal to V_(S)/n when the switch 200 is off. Current I_(S) is then zero because the switch 200 is off because the stages 210 ₁ through 210 _(n) are in a non-conducting (off) state. The voltage ΔV_(S1) through ΔV_(Sn) across each stage 210 ₁ through 210 _(n) when the switch 200 is turned on is, respectively V_(Saturation-1) through V_(Saturation-n), the voltages across each of the semiconductor devices S₁ through S_(n) while in saturation. Current I_(S) is then non-zero because the switch 200 is on because all of the stages 210 ₁ through 210 _(n) are conducting, i.e., in an on state. The switch 200 is not turned on until each stage 210 ₁ through 210 _(n) is conducting (turned on).

Turn-on of the control-triggered stage 210 ₁ begins with each of the semiconductor devices S_(1,1) through S_(1,m) in a non-conducting (off) state such that the stage 210 ₁ is in a non-conducting (off) state. The control-triggered stage 210 ₁ operates as follows during turn-on:

a) In response to a control signal, for example supplied to the switch SW₁, the switch SW₁ across the breakover switch BOS₁ is closed, thereby shorting the first side of the breakover switch BOS₁ to its second side.

b) The voltage drop ΔV_(S1) across the stage 210 ₁ instantaneously decreases slightly and is redistributed over the resistor R₁ and the suppressor Z₁.

c) The voltage at the control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m) instantaneously increases, i.e., a positive voltage pulse is provided to control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m).

d) The positive voltage pulse at the control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m) turns on the semiconductor devices S_(1,1) through S_(1,m) and places them into saturation. The control-triggered stage 210 ₁ is thereby turned on.

Because the semiconductor devices S_(1,1) through S_(1,m) turn on and go into saturation, the voltage at the power inputs PI_(1,1) through PI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m) decreases, thereby pulling down the voltage at the input 212 ₁ of the control-triggered stage 210 ₁ and at the output 211 ₂ of the auto-triggered stage 210 ₂. The voltage ΔV_(S1) across the stage 210 ₁ is V_(Saturation-1) after turn-on. Because the remaining stages 210 ₂ through 210 _(n) are still in the off state and the total voltage V_(S) across the switch 200 remains the same, the voltages ΔV_(S2) through ΔV_(Sn) across respective stages 210 ₂ through 210 _(n) increase after the switch SW₁ closes and the semiconductor devices S_(1,1) through S_(1,m) turn on.

After the stage 210 ₁ is turned on, the voltage ΔV_(S2) through ΔV_(Sn) across each respective stage 210 ₂ through 210 _(n) is (V_(S)-V_(Saturation-1))/(n-1). In other words, the voltage ΔV_(S2) through ΔV_(Sn) across each respective stage 210 ₂ through 210 _(n) increases to (V_(S)-V_(Saturation-1))/(n-1), where (V_(S)-V_(Saturation-1))/(n-1)>V_(S)/n because V_(Saturation-1)<V_(S)/n. This increase in voltage ΔV_(S2) through ΔV_(Sn) across each respective stage 210 ₂ through 210 _(n) causes the stages 210 ₂ through 210 _(n) to turn on by one of two ways: via turn-on of the breakover switches BOS₂ through BOS_(n) or via the capacitors C₂ through C_(n) discharging into the control inputs of their respective semiconductor devices.

Following turn-on of the control-triggered stage 210 ₁, if the voltage ΔV_(S2) through ΔV_(Sn) across each of the respective stages 210 ₂ through 210 _(n) causes the voltage ΔV_(BOS2) through ΔV_(BOSn) across each respective breakover switch BOS₂ through BOS_(n) to exceed the breakover voltage (V_(S)/n+ΔV) of each of the breakover switches BOS₂ through BOS_(n), turn-on of the auto-triggered stages 210 ₂ through 210 _(n) proceeds as follows (“the first auto turn-on procedure”):

a) Each of the breakover switches BOS₂ through BOS_(n) begins conducting at the same time.

b) ΔV_(BOS2) through ΔV_(BOSn) decreases because breakover switches BOS₂ through BOS_(n) begin conducting.

c) The voltage drops ΔV_(S2) through ΔV_(Sn) across the respective stages 210 ₂ through 210 _(n) instantaneously redistribute over the respective resistors R₂ through R_(n) and the suppressors Z₂ through Z_(n).

d) The voltages at the control inputs of the semiconductor devices S₂ through S_(n) instantaneously increase, i.e., positive voltage pulses are provided to control inputs of the semiconductor devices S₂ through S_(n).

e) The positive voltage pulses at the control inputs CI_(2,1) through CI_(2,m) of the semiconductor devices S₂ through S_(n) turn on the semiconductor devices S₂ through S_(n) and place them into saturation.

f) Because the semiconductor devices S₂ through S_(n) turn on and go into saturation, the voltages at the power inputs of the semiconductor devices S₂ through S_(n) decrease, thereby pulling down the respective voltages at the respective inputs 212 ₂ through 212 _(n) of the respective stages 210 ₂ through 210 _(n). The voltages ΔV_(S2) through ΔV_(Sn) across the respective stages 210 ₁ through 210 _(n) are, respectively, V_(Saturation-2) through V_(Saturation-n) after turn-on.

g) The voltage ΔV_(S) across the switch 200 decreases to V_(Saturation-1)+V_(Saturation-2) + . . . +V_(Saturation-n) and I_(S) increases from zero.

Following turn-on of the control-triggered stage 210 ₁, if the voltage ΔV_(S2) through ΔV_(Sn) across each of the respective stages 210 ₂ through 210 _(n) does not cause the voltage ΔV_(BOS2) through ΔV_(BOSn) across each respective breakover switch BOS₂ through BOS_(n) to exceed the breakover voltage (V_(S)/n+ΔV) of each of the breakover switches BOS₂ through BOS_(n), turn-on of the auto-triggered stage 210 ₂ proceeds as follows (“the second auto turn-on procedure”):

a) After the control-input stage 210 ₁ turns on, the voltage ΔV_(S1) decreases to V_(Saturation-1) and the voltages ΔV_(S2) through ΔV_(Sn) across the remaining stages increase to (V_(S)-V_(Saturation-1))/(n-1).

b) The voltages across the resistor R₂, the breakover switch BOS₂, and the suppressor Z₂ increase.

c) The capacitor C₂ discharges into the control inputs CI_(2,1) through CI_(2,m) of the semiconductor devices S_(2,1) through S_(2,m).

d) The semiconductor devices S_(2,1) through S_(2,m) turn on and go into saturation. The auto-triggered stage 210 ₂ is thereby turned on.

Because the semiconductor devices S_(2,1) through S_(2,m) turn on and go into saturation, the voltage at the power inputs PI_(2,1) through PI_(2,m) of the semiconductor devices S_(2,1) through S_(2,m), decrease, thereby pulling down the voltage at the input 212 ₂ of the auto-triggered stage 210 ₂ and at the output 211 ₃ of the auto-triggered stage 210 ₃. The voltage ΔV_(S2) across the stage 210 ₂ is V_(Saturation-2) after turn-on. Because the remaining stages 210 ₃ through 210 _(n) are still in the off state and the total voltage V_(S) across the switch 200 remains the same, the voltages ΔV_(S3) through ΔV_(Sn) across respective stages 210 ₃ through 210 _(n) increase after the auto-triggered stage 210 ₂ turns on.

After the auto-triggered stage 210 ₂ turns on according to the second auto turn-on procedure, the capacitor C₃ begins to discharge into the control inputs CI_(3,1) through CI_(3,m) of the semiconductor devices S_(3,1) through S_(3,m). The auto-triggered stages 210 ₃ through 210 _(n) start to cascade on according to the second auto turn-on procedure. However, turn-on of any of the auto-triggered stages 210 ₃ through 210 _(n) progresses according to the first turn-on procedure if the voltages across the remaining breakover switches are exceeded. Thus, cascading turn-on may shift to simultaneous turn-on once the breakover voltages of the remaining breakover switches are exceeded.

The suppressors Z₁ through Z_(n) provide protection for the control-input-to-power-output junctions of the semiconductor devices S₁ through S_(n) against overcurrent and overvoltage during switching of the switch 200 on and off. The breakover switches BOS₁ through BOS_(n) also provide protection for each of the levels 210 ₁ through 210 _(n) from overvoltage when the switch 200 is off and while it is being turned on. Protection from overvoltage during turn-on is desirable as the voltages across levels that are not on increase as other levels are turned on. Thus, the switch 200 is able to handle voltages V_(S) in the level of 10 s of kilovolts.

As noted above, the semiconductor devices S₁ through S_(n) are desirably distributed m per stage, although differing numbers of semiconductor devices per stage are contemplated. Thus, the current I_(S) is divided across m semiconductors in each of stages 210 ₁ through 210 _(n). Accordingly, the switch 200 is able to handle currents I_(S) in the level of 10s of kiloamps.

Turn-off of the switch 200 proceeds as follows:

a) In response to a control signal, for example supplied to the turn-off circuit 215, the turn-off circuit 215 provides a negative-voltage signal to the control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m), for example by pulling the control inputs to ground 140.

b) The negative voltage pulse at the control inputs CI_(1,1) through CI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m) turns of the semiconductor devices S_(1,1) through S_(1,m).

c) Because the semiconductor devices S_(1,1) through S_(1,m) turn off, the voltage at the power inputs PI_(1,1) through PI_(1,m) of the semiconductor devices S_(1,1) through S_(1,m) increases, thereby causing the voltage at the input 212 ₁ of the control-triggered stage 210 ₁ and at the output 211 ₂ of the auto-triggered stage 210 ₂ to rise. The voltage ΔV_(S1) across the stage 210 ₁ increases, and the voltage ΔV_(S2) across the stage 210 ₂ decreases.

d) Negative current from the control inputs CI_(2,1) through CI_(2,m) of the semiconductor devices S_(2,1) through S_(2,m) charges the capacitor C₁ providing a negative-voltage signal to the control inputs CI_(2,1) through CI_(2,m) of the semiconductor devices S_(2,1) through S_(2,m).

e) The semiconductor devices S_(2,1) through S_(2,m) turn off.

f) Steps (c) through (e) repeat for each next higher stage, so that the stages 210 ₃ through 210 _(n) cascade off until all of the stages of the switch 200 are turned off.

Turn-on and turn-off of the switch 200 does not require the use of a pulse transformer for each stage 210 ₁ through 210 _(n). As noted above, pulse transformers are bulky and require isolation from one another, increasing the size and complexity of the switch. Further, pulse transformers have non-negligible inductance, which is not desirable for high voltage, high current switches. Finally, pulse transformers provide pulses of short time periods, which may not be long enough to turn on the semiconductor devices S ₁ through S_(n).

By using breakover switches BOS₁ through BOS_(n) and capacitors connecting the output of one level to the control inputs of the semiconductor devices of another lover, the switch 200 provides for very fast turn-on and turn-off and avoids the use of pulse transformers. Inductance in the switch 200 is reduced and the control inputs of the semiconductor devices S₁ through S_(n) may be kept at a negative voltage for as long as is needed to turn them on. For a six level device, turn-on time may be 200 nanoseconds.

Various types of semiconductor devices are contemplated for the semiconductor devices S₁ through S_(n). For example, in an exemplary embodiment, the semiconductor devices S₁ through S_(n) are IGBTs. In another embodiment, they are MOSFETs. In yet another embodiment, they are SCRs. In still another embodiment, they are gate turn-off thyristors (GTOs). In a further embodiment, they are MOS controlled thyristors or light-controlled thyristors. It is to be understood that the anodes or collectors of such devices are the power inputs, as that term is used herein, the cathodes or emitters of such devices are the power outputs, as that term is used herein, and the gates of such devices are the control inputs, as that term is used herein.

The semiconductor devices S₁ through S_(n) desirably have low output impedances when on. For example, in an exemplary embodiment, the semiconductor devices S₁ through S_(n) desirably have output impedances in the milliohm region, and, when off, they desirably have output impedances greater than 5,000 ohms. The input impedances of the semiconductor devices S₁ through S_(n) depend on the type of semiconductor device employed. If the semiconductor devices S₁ through S_(n) are MOSFETs, then the input impedances are in the megohm region. If they are thyristors, then the input impedances are in the milliohm region.

In the embodiments of the switch 200 described herein, the switch 200 may include the suppressors Z₁ through Z_(n) to protect the semiconductor devices S₁ through S_(n) against overcurrent and overvoltage during turn-on and turn-off. In an exemplary embodiment, the semiconductor devices S₁ through S_(n) include gate-to-cathode junctions, and the suppressors Z₁ through Z_(n) provide protection for these gate-to-cathode junctions against overcurrent and overvoltage during switching of the switch 200 on and off. It is to be understood that other embodiments of the switch 200 are contemplated in which the suppressors Z₁ through Z_(n) are not included.

It is further to be understood that the breakover switches BOS₁ through BOS_(n) also provide protection for each of the levels 210 ₁ through 210 _(n) from overvoltage when the switch 200 is off and while it is being turned on. Protection from overvoltage during turn-on is desirable as the voltages across levels that are not on increase as other levels are turned on. Thus, the switch 200 is able to handle voltages V_(S) in the level of 10 s of kilovolts.

In an exemplary embodiment, the energy storage device 120 is a capacitor. FIG. 3 illustrates the waves for the voltage V_(S) across an exemplary embodiment of the energy storage device 120 and the current I_(S) supplied by the exemplary energy storage device 120 as an exemplary embodiment of the switch 200 is turned on, in accordance with an exemplary embodiment of the present invention. For the example illustrated in FIG. 3, the energy storage device 120 is a 0.15 μF capacitor.

Referring to FIG. 3, prior to the exemplary switch 200 closing, V_(S) is 4.92 kV and I_(S) is 0 A. At time t₀, corresponding to 0 seconds, a command signal is sent to the switch SW₁ to close, beginning the turn-on process of the exemplary switch 200. At time t₁, corresponding to 40 ns, the exemplary switch 200 is fully turned on, and the exemplary energy storage device 120 begins discharging. I_(S) increases to a maximum of 2.04 kA at time t₂, corresponding to 48 ns. V_(S) and I_(S) then oscillate until settling at zero.

FIG. 4 illustrates the waves for the voltage V_(S) across another exemplary embodiment of the energy storage device 120 and the current I_(S) supplied by the exemplary energy storage device 120 as another exemplary embodiment of the switch 200 is turned on and then turned off, in accordance with an exemplary embodiment of the present invention. In FIG. 4, prior to the exemplary switch 200 closing, V_(S) is about 1.4 kV and I_(S) is 0 A. At time t₁, the exemplary switch 200 is fully turned on, and the exemplary energy storage device 120 begins discharging. I_(S) increases to a maximum of 100 A at time t₂, at which time the exemplary switch 200 is provided with a turn-off signal via the turn-off circuit 215. The exemplary switch 200 turns off, as described above. As the exemplary switch 200 turns off, I_(S) decreases, and V_(S) spikes to 5.4 kV at time t₃ before returning to 1.4 kV when the exemplary switch 200 is fully closed.

These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it is to be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It is to be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention. 

What is claimed is:
 1. A semiconductor switching device comprising: a control-triggered stage comprising: one or more semiconductor switches connected in parallel, each semiconductor switch comprising a power input, a power output, and a control input; a breakover switch comprising a first end and a second end, the first end of the breakover switch coupled to the power input of each semiconductor switch, the second end of the breakover switch coupled to the control input of each semiconductor switch; and a control switch connected across the breakover switch; one or more auto-triggered stages connected in series, each auto-triggered stage comprising: one or more semiconductor switches connected in parallel, each semiconductor switch of each auto-triggered stage comprising a power input, a power output, and a control input; and a breakover switch comprising a first end and a second end, the first end of the breakover switch of the each auto-triggered stage coupled to the power input of each semiconductor switch of the each auto-triggered stage, the second end of the breakover switch of the each auto-triggered stage coupled to the control input of each semiconductor switch of the each auto-triggered stage, wherein the control-triggered stage is connected in series with the one or more auto-triggered stages.
 2. The semiconductor switching device of claim 1, wherein: the control-triggered stage further comprises a resistor; and the first end of the breakover switch of the control-triggered stage is coupled to the power input of each semiconductor switch of the control-triggered stage via the resistor of the control-triggered stage.
 3. The semiconductor switching device of claim 2, wherein: each of the auto-triggered stages comprises a respective resistor; and the first end of the breakover switch of the each auto-triggered stage is coupled to the power input of each semiconductor switch of the each auto-triggered stage via the respective resistor of the each auto-triggered stage.
 4. The semiconductor switching device of claim 1, wherein each breakover switch is a breakover diode.
 5. The semiconductor switching device of claim 1, wherein the control-triggered stage further comprises a suppressor having a first end and a second end, the first end of the suppressor coupled to the power output of each semiconductor switch of the control-triggered stage and the second end of the suppressor coupled to the control input of each semiconductor switch of the control-triggered stage.
 6. The semiconductor switching device of claim 5, wherein the control-triggered stage further comprises a turn-off circuit disposed between the power output and the control input of each semiconductor switch of the control-triggered stage.
 7. The semiconductor switching device of claim 5, wherein: each of the auto-triggered stages comprises a respective suppressor having a first end and a second end; the first end of the respective suppressor of each auto-triggered stage is coupled to the power output of each semiconductor switch of the each auto-triggered stage; and the second end of the respective suppressor of each auto-triggered stage is coupled to the control input of each semiconductor switch of the each auto-triggered stage.
 8. The semiconductor switching device of claim 1, wherein the control-triggered stage further comprises a capacitor having a first end and a second end, the first end of the capacitor coupled to the power output of each semiconductor switch of the control-triggered stage and the second end of the capacitor coupled to the control input of each semiconductor switch of one of the auto-triggered stages, the capacitor configured to discharge into the control input of the each semiconductor switch of the one of the auto-triggered stages in response to the control switch closing to turn on the each semiconductor switch of the one of the auto-triggered stages.
 9. The semiconductor switching device of claim 8, wherein: each of the auto-triggered stages comprises a respective capacitor having a first end and a second end; the first end of the respective capacitor of each auto-triggered stage is coupled to the power output of each semiconductor switch of the each auto-triggered stage; the second end of the respective capacitor of each auto-triggered stage except a highest one of the auto-triggered stages is coupled to the control input of each semiconductor switch of a next higher auto-triggered stage; and the second end of the capacitor of the highest auto-triggered stage is coupled to the power input of each semiconductor switch of the highest auto-triggered stage.
 10. The semiconductor switching device of claim 1, wherein each semiconductor switch has a low output impedance.
 11. The semiconductor switching device of claim 10, wherein each semiconductor switch is an insulated gate bipolar transistor.
 12. The semiconductor switching device of claim 10, wherein each semiconductor switch is a silicon controlled rectifier.
 13. The semiconductor switching device of claim 10, wherein each semiconductor switch is a gate-turn-off thyristor.
 14. The semiconductor switching device of claim 10, wherein each semiconductor switch is a metal-oxide semiconductor field-effect transistor.
 15. The semiconductor switching device of claim 10, wherein each semiconductor switch is a MOS controlled thyristor.
 16. A high voltage circuit comprising: an energy storage device; a load; a power supply coupled across the energy storage device; and a semiconductor switching device for coupling the energy storage device across the load, the semiconductor switching device comprising: a control-triggered stage comprising: one or more semiconductor switches connected in parallel, each semiconductor switch comprising a switch input, a switch output, and a control input; a breakover switch comprising an input and an output, the input of the breakover switch coupled to the switch input of each semiconductor switch, the output of the breakover switch coupled to the control input of each semiconductor switch; and a control switch connected across the breakover switch; one or more auto-triggered stages connected in series, each auto-triggered stage comprising: one or more semiconductor switches connected in parallel, each semiconductor switch of each auto-triggered stage comprising a switch input, a switch output, and a control input; and a breakover switch comprising an input and an output, the input of the breakover switch of the each auto-triggered stage coupled to the switch input of each semiconductor switch of the each auto-triggered stage, the output of the breakover switch of the each auto-triggered stage coupled to the control input of each semiconductor switch of the each auto-triggered stage, wherein the control-triggered stage is connected in series with the one or more auto-triggered stages.
 17. The high voltage circuit of claim 16, wherein: the control-triggered stage further comprises a resistor; and the first end of the breakover switch of the control-triggered stage is coupled to the power input of each semiconductor switch of the control-triggered stage via the resistor of the control-triggered stage.
 18. The high voltage circuit of claim 17, wherein: each of the auto-triggered stages comprises a respective resistor; and the first end of the breakover switch of the each auto-triggered stage is coupled to the power input of each semiconductor switch of the each auto-triggered stage via the respective resistor of the each auto-triggered stage.
 19. The high voltage circuit of claim 16, wherein each breakover switch is a breakover diode.
 20. The high voltage circuit of claim 16, wherein the control-triggered stage further comprises a suppressor having a first end and a second end, the first end of the suppressor coupled to the power output of each semiconductor switch of the control-triggered stage and the second end of the suppressor coupled to the control input of each semiconductor switch of the control-triggered stage.
 21. The high voltage circuit of claim 20, wherein the control-triggered stage further comprises a turn-off circuit disposed between the power output and the control input of each semiconductor switch of the control-triggered stage.
 22. The high voltage circuit of claim 20, wherein: each of the auto-triggered stages comprises a respective suppressor having a first end and a second end; the first end of the respective suppressor of each auto-triggered stage is coupled to the power output of each semiconductor switch of the each auto-triggered stage; and the second end of the respective suppressor of each auto-triggered stage is coupled to the control input of each semiconductor switch of the each auto-triggered stage.
 23. The high voltage circuit of claim 16, wherein the control-triggered stage further comprises a capacitor having a first end and a second end, the first end of the capacitor coupled to the power output of each semiconductor switch of the control-triggered stage and the second end of the capacitor coupled to the control input of each semiconductor switch of one of the auto-triggered stages, the capacitor configured to discharge into the control input of the each semiconductor switch of the one of the auto-triggered stages in response to the control switch closing to turn on the each semiconductor switch of the one of the auto-triggered stages.
 24. The high voltage circuit of claim 23, wherein: each of the auto-triggered stages comprises a respective capacitor having a first end and a second end; the first end of the respective capacitor of each auto-triggered stage is coupled to the power output of each semiconductor switch of the each auto-triggered stage; the second end of the respective capacitor of each auto-triggered stage except a highest one of the auto-triggered stages is coupled to the control input of each semiconductor switch of a next higher auto-triggered stage; and the second end of the capacitor of the highest auto-triggered stage is coupled to the power input of each semiconductor switch of the highest auto-triggered stage.
 25. The high voltage circuit of claim 16, wherein each semiconductor switch has a low output impedance. 